llvm dev - Feb 2014 - [LLVMdev] [cfe-dev] Unwind behaviour in Clang/LLVM

>> The semantics uwtable is just "make sure it is possible to unwind
past
>> this function". It doesn't include anything more,
>> like the ability to run destructors. It is used for ABIs that require
>> it for use in debuggers and profilers.
>
> Well, that meaning fails when you have both uwtable and nounwind,
> which means: "generate a CantUnwind table".
Why fails? I would read a "generate a CantUnwind table" as a table
that a debugger or profiler can use, but lacks information to allow an
exception to be handled.
>> I am not too sure about nounwind. I think it is more about using
>> unwind for control flow in the language. Having it allows us
>> to prune information used for exception handling.
>
> nounwind translates into "doesNotThrow()", which is *also* used
on
> "needsUnwindTables()" which is used in
"needsCFIMoves()".
>
> It's not simple. ;)
>
> cheers,
> --renato
Cheers,
Rafael

On 11 February 2014 14:12, Rafael Espíndola <rafael.espindola at
gmail.com> wrote:
> Why fails? I would read a "generate a CantUnwind table" as a
table
> that a debugger or profiler can use, but lacks information to allow an
> exception to be handled.
Right, I was assuming CantUnwind was interpreted as is by debug and
profilers. If CantUnwind is *only* blocking EH, than it looks good.

Let Keith's patch go in and I'll have a go trying to merge the two.

Thanks,
--renato

I assume this "CantUnwind table" is not the same as

Unwind table index '.ARM.exidx' at offset 0x818 contains 4 entries:
0x5d4 <main>: 0x1 [cantunwind]

because the latter prevent any unwinding, breaking
debuggers/profilers/sanitizers.

As I understand, the right way to enable basic unwind through any
function (without emitting landing pands etc, unless they are needed
for something else) is -funwind-tables. I'm going to add that in the
driver for all sanitizers then.


On Tue, Feb 11, 2014 at 6:20 PM, Renato Golin <renato.golin at linaro.org>
wrote:
> On 11 February 2014 14:12, Rafael Espíndola <rafael.espindola at
gmail.com> wrote:
>> Why fails? I would read a "generate a CantUnwind table" as a
table
>> that a debugger or profiler can use, but lacks information to allow an
>> exception to be handled.
>
> Right, I was assuming CantUnwind was interpreted as is by debug and
> profilers. If CantUnwind is *only* blocking EH, than it looks good.
>
> Let Keith's patch go in and I'll have a go trying to merge the two.
>
> Thanks,
> --renato
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev